The present invention is related to external pipe or vessel dilation sensing apparatus and more particularly to such apparatus for measuring pressure differential through detection of strain on the external circumferential surface of the pipe or vessel.
It is desirable, particularly in the nuclear power and testing industry as well as other industries, to monitor low level pressure fluctuations within fluid conduits. Pressure monitoring can be accomplished from within the conduit by standard measuring techniques. However, such techniques require that there be access to the interior of the conduit. Safety factors, shut down time required to install or maintain standard measuring equipment, strength reduction of the conduit walls and the caustic or toxic nature of the conduit contents are all valid reasons for not using internal pressure monitoring in otherwise closed systems. The present apparautus provides accurate pressure monitoring while assuring the system integrity.
The usual technique used to externally monitor pressure fluctuations within a conduit involves mounting one or more electrically resistive strain gauges on the external surface of the conduit. Expansion of the surface due to pressure fluctuations changes the electrical resistance through the strain gauges. Measurements of the change in resistance can easily be related to the strain and to the internal pressure causing the strain. This technique is serviceable and widely used where monitoring of relatively large pressure fluctuations is desired. However, a strain gauge measures only the strain at the point of contact between itself and the pipe circumference. Low pressure variations cause such insignificant changes in the resistance of the gauge as to defy accurate measurement. A problem is thus encountered when it is desirable to measure small pressure variations, say in the range of 3 psi. In addition, it may not be possible to use conventionally attached strain gauges on piping that has a temperature in excess of 500.degree. F.
The present apparatus overcomes this shortcoming by use of two rigid beams clamped across the external conduit diameter and supporting a sensitive strain gauge between them at one side of the conduit. The beams are fixed spatially at the opposite conduit side. The beam lengths mechanically magnify any diametric size variations in the conduit, as detected by the strain gauge.